Enable the -Wundef warning for clang

webrtc/modules/audio_coding/neteq/tools/input_audio_file_unittest.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 // Unit tests for test InputAudioFile class.
 
 #include "webrtc/modules/audio_coding/neteq/tools/input_audio_file.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 namespace test {
 
 TEST(TestInputAudioFile, DuplicateInterleaveSeparateSrcDst) {
   static const size_t kSamples = 10;
   static const size_t kChannels = 2;
   int16_t input[kSamples];
   for (size_t i = 0; i < kSamples; ++i) {
     input[i] = i;
@@ skipping 23 matching lines @@
   int16_t* output_ptr = input;
   for (size_t i = 0; i < kSamples; ++i) {
     for (size_t j = 0; j < kChannels; ++j) {
       EXPECT_EQ(static_cast<int16_t>(i), *output_ptr++);
     }
   }
 }
 
 }  // namespace test
 }  // namespace webrtc